Method for producing methane gas

Abstract

A practical technique that enables effective utilization of organic wastes is provided. In order to achieve the above-mentioned object, a method for producing methane gas from organic wastes according to the present invention includes: treating organic wastes with at least one of supercritical water and sub-critical water to convert the organic wastes into low molecular weight substances; and subjecting the low molecular weight substances to methane fermentation. According to the method, initially, the organic wastes are treated with at least one of the supercritical water and the sub-critical water so as to be converted into low molecular weight substances that are easily subjected to methane fermentation. Then, the treated substances are subjected to methane fermentation. Consequently, methane gas can be produced from the organic wastes at a high speed with high digestion efficiency.

1. A method for producing methane gas from organic wastes, comprising:
 (a) treating organic wastes with at least one of supercritical water and sub-critical water to convert the organic wastes into low molecular weight substances; 
 (b) separating the treated substances into an oil phase, a water phase, and a solid phase, so that the water phase is collected; 
 (c) feeding the water phase collected in step (b) into a methane fermentation vessel; and 
 (d) subjecting the collected water phase to methane fermentation within the methane fermentation vessel, 
 wherein in step (a), the organic wastes treated with at least one of supercritical water and sub-critical water are organic wastes that have not been subjected to methane fermentation within the methane fermentation vessel. 
 
     
     
       2. The method according to  claim 1 , wherein the treatment for conversion into low molecular weight substances is a treatment with sub-critical water. 
     
     
       3. The method according to  claim 1 , wherein in the treatment with sub-critical water, a treatment temperature is 440 to 553 K, and a treatment pressure is 0.8 to 6.4 MPa. 
     
     
       4. The method according to  claim 1 , wherein a time taken for the treatment for conversion into low molecular weight substances is 1 to 20 minutes. 
     
     
       5. The method according to  claim 1 , wherein the treatment for conversion into low molecular weight substances is performed continuously. 
     
     
       6. The method according to  claim 1 , wherein the water phase contains organic acid. 
     
     
       7. The method according to  claim 6 , wherein the organic acid includes acetic acid. 
     
     
       8. The method according to  claim 1 , wherein a time for the methane fermentation is in a range of 5 to 48 hours. 
     
     
       9. The method according to  claim 1 , wherein carbon digestion efficiency in the methane fermentation is 90% or more. 
     
     
       10. The method according to  claim 1 , wherein the organic waste is activated sludge. 
     
     
       11. The method according to  claim 1 , further comprising separating and collecting a useful material generated in the treatment for conversion into low molecular weight substances. 
     
     
       12. The method according to  claim 11 , wherein the useful material generates at least one of phosphoric acid, organic acid, fatty acid, amino acid, and sugar. 
     
     
       13. The method according to  claim 11 , wherein by adjusting at least one of a treatment temperature and a treatment time in the treatment for conversion into low molecular weight substances, the useful material is allowed to be generated selectively.